From 1998 to 2000, 212 isolates of Phytophthora infestans were collected in tomato (Lycopersicon esculentum) and potato (Solanum tuberosum) fields of the Zona da Mata region of the State of Minas Gerais, Brazil. Samples were taken from 41 commercial tomato and ten experimental potato fields. Isolates were characterized for mating type, glucose 6-phosphate-isomerase alloenzyme (Gpi), metalaxyl resistance using the leaf disc method, and pathogenicity and virulence. All isolates were of the A1-mating type. Of the 96 isolates tested for Gpi, 95 had the 86/100 phenotype, typical of the US-1 clonal lineage, and one was of the 100/100 genotype. In 1998, the frequency of isolates sensitive (S), intermediate (I), or resistant (R) to metalaxyl was 40%, 40%, and 20%, respectively. In 2000 there was a change in the frequency in each class of metalaxyl resistance: 3.2% (S); 61.3% (I); and 35.5% (R). All isolates were virulent when inoculated in tomato 'Kada' and potato 'Bintje'. The majority of the isolates were virulent on tomato containing Ph1 (91%) or Ph2 (95%) genes. Eight isolates were virulent to a potato clone containing R1 gene; and 15 were virulent on a R11 clone. Virulence on potato clones carrying genes R1 and R11 was much lower than on tomato cultivars carrying genes Ph1 and Ph2. The population of P. infestans in the Zona da Mata region seems to be comprised of isolates of the US-1 clonal lineage, with high race variability and low sensitivity to metalaxyl.
